Ground rodents can cause major damage to lawns, golf links, racecourses and farms. These animals are burrowers. They feed on the earthworms and insects they find in their tunnels and in the earth. They need to consume about half their own weight daily, thus they are extremely active in their search for food. They use their powerful and efficient front feet as shovels to create a network of underground tunnels. This burrowing often damages the root systems of landscaping shrubs and trees, killing young specimens or causing them to lean; it also damages crops, flower bulbs, and root vegetables. The soil that is excavated during this process is often left as unsightly mounds. In addition, some species create unattractive runways in lawns by eating grass blades and passing numerous times over the same path. Molehills and tunnel holes can trip up horses and cattle. Stones thrown up by the moles can wreck farm machinery. The tunneling may also damage plastic water lines and other underground installations. Abandoned tunnels often divert water leading to soil erosion.
Moles and the like have acute hearing and highly sensitive noses and tails. Sending sonic signals into the ground has been shown to be effective in decreasing ground rodent activity. It is thought that sonic pulses in the ground are offensive to the animals' acute hearing and sense of touch and that such action interferes with their food-finding abilities. When disturbed to a great enough extent, they will leave the area of the in-ground sonic activity to seek more hospitable environment.
The prior art includes a number of references which disclose devices utilizing sonic waves to repel ground burrowing vermin. They all utilize, in some configuration, a removable battery chamber to which is also attached the circuitry and sonic generating means. Among these references are U.S. Pat. Nos. 5,205,066; 5,822,917; 5,870,847; and 6,157,594. The method of use of these devices is the same. The devices are switched on and inserted into the ground to generate sound waves through the area. They typically remain in the ground for an extended period of time, during which devices become covered with dirt and other debris. When the battery wears out, the person charged with the maintenance of the system has to locate the device, which often requires digging into the ground by hand. Then he has to remove the dirt-covered cap, and lift out the interior frame which contains the batteries and the sonic generating equipment. He then replaces the spent batteries, inserts the frame back into the housing, and the tightens the cap. All this manipulation of the interior workings of the device is usually accomplished with dirt-covered hands. It is during this battery changing event that the main drawback of these devices is triggered: the sonic generating equipment becomes exposed to and damaged by dirt and the elements.
Another drawback to the prior art designs is the reduced efficiency of sonic transmission due to the fact that the sonic generating equipment is attached to the battery chamber. Because the battery chamber is removable, i.e., not fixed to the housing, some of the energy produced by the sonic generator is absorbed by the battery chamber, and therefore dissipated within the housing.